Background: Early diastolic mitral valve annulus velocity (E') is one of parameters representing diastolic left ventricular (LV) function. However, the property of E��� is not well characterized. Irregular RR intervals in atrial fibrillation (AF) make beat to beat changes in systolic LV performances. We investigated the influence of RR intervals on E��� in AF. Methods: One hundred and forty-three AF patients were screened. Patients with stenotic valvular diseases, pericardial diseases or acute conditions were excluded, and 117 patients were enrolled. The relation between preceding RR interval (RR-1) or pre-preceding RR interval (RR-2) and E' were obtained using the logarithmic equation, from which the slope was calculated. Results: The slope in the relation between RR-1 and E' varied from -2.3 to 2.7 (figure). The slope was lower (more likely negative) in patients with ischemic heart disease (IHD, r=0.22, p=0.02), higher E��� to Early diastolic mitral flow velocity (E) ratio (r=-0.21, p=0.02) and smaller diastolic LV dimension (r=0.19, p=0.04). When patients were divided into three groups according to the slope, the lowest slope group (<-0.55, n=40) was associated with IHD, higher E���/E and no heart failure history compared with the highest slope group (>0.57, n=39). The slope in the relation between RR-2 and E' also varied from -3.4 to 3.1. The slope was lower in patients with larger systolic LV dimension (r=-0.38, p<1.001) and lower E (r=0.30, p=0.001). Conclusion: In summary, beat to beat changes in systolic function derived from irregular RR interval in AF had variable effects on diastolic function according to clinical variables. Low heart rate may be not beneficial in AF patients with IHD, high end-diastolic pressure and small chamber size. This finding suggests that target heart rate in AF should be individualized. Considering the weak influence of clinical variable, the relation between RR interval and E��� might be helpful to determine individual target heart rate.